Goal and Hypothesis: During cancer progression, the cells encounter many stress signals. If beyond repair, the cells have built-in mechanisms to eliminate themselves. The successful cancer cells managed to foil this hardwired stress response. In fact, it appears that cancer cells can co-opt some tumor suppressors to become oncogenes. TGF[unreadable] is the best-known example exhibiting this "Jekyll and Hyde" conversion. ATF3, a stress- inducible gene, is a regulatory gene that was recently identified to have a dichotomous role in cancer progression: it is pro-apoptotic in non-transformed breast epithelial cells, but protects the malignant cells from stress and promotes their metastasis. The long-term objective is to understand the cancer dichotomy using ATF3 as a handle to address this issue. This proposal will focus on the oncogenic aspect of ATF3 in breast cancer. Aim 1 will test the hypothesis that the interaction of ATF3 with Smad3, a protein in the TGF[unreadable] pathway, plays an important role in the oncogenic activity of ATF3 in advanced breast cancer cells. This will be tested by structure-function analyses, including domain swap and site-directed mutagenesis. Aim 2 will test the hypothesis that ATF3 exerts its oncogenic action in malignant cells, at least in part, by regulating downstream target genes. Potential target promoters will be tested by chromatin immunoprecipitation assay and transcription assay to determine whether they are the direct target genes of ATF3. In addition, the biological significance of their regulation by ATF3 will be tested. Aim 3 will test the hypothesis that ATF3 is important for macrophage-cancer interaction. Both gain- and loss- of-function approaches, using in vitro co-culture and in vivo fat pad injection models, will be taken to test whether ATF3 plays a role in the ability of cancer cells to interact with macrophages. Significance: ATF3 is a new regulator that has a dichotomous role in cancer progression, and may play a role in stroma-cancer interaction. Because it is induced by anti-cancer drugs, its oncogenic function indicates that these drugs may have undesired effects. Information from the proposal may provide clues for rational designs of anti-cancer treatment, thus potentially changing the clinical practice in the future. PUBLIC HEALTH RELEVANCE: Despite the tremendous advances in early detections and treatments, breast cancer becomes incurable once metastasized beyond the regional lymph nodes. Thus, to combat breast cancer, it is essential to better understand its metastasis. This proposal investigates a master switch gene that regulates breast cancer metastasis.